Viking

Twin spacecraft, Viking 1 and Viking 2, launched toward Mars
on Aug. 20 and Sep. 8, 1975. Each consisted of an orbiter and a lander (with
a combined mass, less propellant, of 600 kg), the latter carrying a sophisticated
package of instruments, including several experiments designed to search
for traces of life in the Martian soil (see Mars,
life).

After arrival in orbit around the planet on Jun. 19 and Aug. 7, 1976 (Viking
1 having departed second but arrived first), a detailed reconnaissance was
carried out to identify suitable and safe landing sites, during which the
United States celebrated its bicentennial. On Jul. 20, the Viking 1 lander
touched down in the western part of Chryse Planitia,
at lat. 22.4°N and long. 47.5°W, followed, on Sep. 3, by its sister
craft 7,420 km to the northeast, in Utopia
Planitia, at latitude 48°N and longitude 226°W. .

Early results

Preliminary tests on the first soil sample, collected by Viking 1 on Jul.
28, 1976, immediately brought surprises. Gas was given off following the
introduction of nutrient in the LR experiment, before the reaction tapered
off – exactly the outcome expected if microorganisms had been present
in the soil. The PR experiment also gave a positive result, compatible with
metabolic activity in the sample. In the case of the GEX experiment, both
carbon dioxide and oxygen were evolved, the latter being a reaction
never before seen in tests on either terrestrial or lunar soils. The speed
and course of the reaction, however, suggested that it might be due to an
oxidizing chemical rather than a biological
process. On the face of it, then, two of the three biology experiments gave
indications of life. But the results of the first GCMS tests were curiously
negative. No organics were detected in the Martian soil to the level
of a few parts in a billion, prompting Viking project scientist, Gerald
A. Soffen, to comment "All the signs suggest that life exists on Mars, but
we can't find any bodies!"

Ten weeks later, following more tests at both landing sites, researchers
were still undecided about what their results meant. Soffen summed up the
general feeling:1

The tests revealed
a surprisingly chemically active surface – very like oxidizing.
All experiments yielded results, but these are subject to wide interpretation.
No conclusions were reached concerning the existence of life on Mars.

Conclusions and unanswered questions

At the end of eight and a half months of investigations on the Martian surface,
including 26 biology experiments, the jury was still out – and remains
so to this day. The failure of the GCMS to find any organics whatsoever
was particularly surprising because even soil and rock samples brought back
from the moon by the Apollo astronauts had some organic molecules, deposited
over the ages by carbonaceous meteorites. Further confounding the issue
is that isotopic evidence for organic molecules indigenous to Mars has been
identified in three Martian meteorites found on Earth.

The GCMS was used as the final court of appeal. Instead of looking at the
possibility that the GCMS instrument itself might have somehow been flawed
or in error, the Viking team began to propose theories as to why no organic
molecules existed on Mars. This was done despite doubts that had been raised
during early testing over the GCMS's ability to detect organic matter even
in soils on Earth.

An "official" position emerged that some highly oxidizing chemical, probably
hydrogen peroxide (H2O2),
was responsible for the activity registered by the biological experiments.
A cogent theory based on this idea was put forward by Vance Oyama.
Hydrogen peroxide could have been formed naturally as a result of ultraviolet
light splitting water molecules in the Martian atmosphere. The hydroxide
(OH) radicals thus created would recombine to form, for example, H2O2).
The peroxide would eventually drift down from the atmosphere and be adsorbed
onto soil grains. Over the eons, the process would literally bleach away
all the organic molecules near the surface, even those from meteorites,
and thereby eliminate any chance for life to exist near the surface.

A planet-wide search for hydrogen peroxide in the Martian atmosphere got
under way, without success. A team of scientists at the Laboratory for Extraterrestrial
Physics at NASA's Goddard Space Flight Center, using a spectrometer mounted
on the Kitt Peak National Observatory 4-meter telescope, found no hydrogen
peroxide present in detectable amounts. The sensitivity of the instrument
reduces the upper limit for hydrogen peroxide in the atmosphere of Mars
to 30 parts per billion.

Even if the oxidant hypothesis proves true, the issue of surface or near-surface
life on Mars would not necessarily be closed. According to one 1989 NASA
Ames Research Center study, by Rocco L. Mancinelli, even if oxidants such
as hydrogen peroxide were present at a level of 25–250 parts per million,
they would not prevent the survival and growth of some terrestrial microorganisms,
let alone any that may have evolved and adapted in the Mars environment.
That inference seems even more likely today because our knowledge of the
extreme conditions under which microbes can survive has grown immensely
since the days of the Viking mission and the Mancinelli study.

A minority of researchers, most notably Gilbert Levin,
continue to argue that the Viking data are better explained in biological
terms. Intense investigation of the Red Planet over the next decade seems
likely to settle this issue once and for all.2, 3, 4, 5. For
more, see Viking's search
for life on Mars